Aluminium is a malleable metal, paramagnetic, of low density, highly conductive and has excellent resistance properties to oxidation.
In fact, during its oxidation with air it forms very rapidly on the surface a layer fine of alumina which protects it from the progression of oxidation. This natural layer of alumina has a thickness generally ranging from one to five micrometers.
However, alumina is highly insulating, as opposed to aluminium, and forms a barrier especially preventing the aluminium from attaching to another metal (for example on iron, steel or copper).
Some applications however require materials having at the same time properties of the aluminium, especially its conductivity, its low density, etc. but also additional properties such as magnetism, resistance to breaking, welding capacity, rigidity, or even better conductivity that steel, nickel or copper especially can present.
The manufacture of an aluminium-based alloy is often highly problematical. Typically, equilibrium diagrams show that aluminium-based alloys with ferromagnetic materials are possible, but they are likely to form a limited number of compounds only, mechanically fragile and paramagnetic.
The manufacture and forming of metal assemblies comprising an object made of aluminium and a coating in another metal, especially nickel, copper, iron, cobalt, etc. also interests manufacturers. In fact, making light and rigid metal assemblies currently requires assembly by welding, adhesion, etc. which cannot easily be done with all types of metals, aluminium in particular, which generally have limited cohesion.
For example, in the field of electrical conductors, the aim is to produce conductors having excellent conductivity, which are robust and durable and resist mechanical stresses to which they can be subjected during installation, but also be light and low-cost.
Copper, which is the metal most commonly used for electric wires, has excellent thermal and electric conductivity and produces robust and durable conductors. But these conductors are dense and cannot be used in all areas, in particular in aeronautics or in space research.
Aluminium however, due to its low density, is particularly adapted in those areas where the mass of the conductor constitutes technical stress. Typically, it is often used in the transport of high-power electricity. Nevertheless, aluminium is less conductive than copper and more malleable such that it breaks more easily and therefore cannot be used in any type of electrical installation. In addition, connections made of aluminium prove difficult, both because of the layer of alumina which forms on the surface of the aluminium and which, as has been seen, prevents any welding and is not conductive. It is therefore often necessary to use specific connection means.
The aim has been therefore to obtain metal assemblies constituted at the same time by aluminium and another metal such as copper, to combine the advantageous properties of these materials. Previously known processes are difficult to implement and are relatively costly.
Typically, chemical or electrolytic deposits of metals on aluminium have a very low yield because of the presence of the layer of alumina.
Another known process consists of projecting the metals by plasma, such as copper, onto the aluminium object. This process is however difficult to implement and is costly.
An aim of the invention is therefore to propose a manufacturing process of an aluminium-metal assembly which is reproducible, low-cost, which involves low energy expenditure and which produces an assembly at the same time exhibiting properties of aluminium and that of another metal.
For this, the invention proposes a manufacturing process of an aluminium-metal metal assembly, comprising a metal layer made of a first metal at the surface of a metal object made of a second metal, the first or the second metal being aluminium, characterised in that it comprises the following steps:                placing a metal foam having an open porosity onto a surface of the metal object, and        applying mechanical stress to the metal foam so as to embed it in the surface of the object.        
Some preferred though non-limiting aspects of the process according to the invention are the following:                the first metal is selected from the following group: copper, nickel, iron, aluminium,        the object is a wire, a cable, a bar, a foil, a sheet metal,        the metal object is a foil or sheet of aluminium, and the mechanical stress is applied to the metal assembly by rolling, press-forming, embossing, forging, or stamping of the metal object with the metal foam,        rolling is done cold,        the object is a wire or a cable and the mechanical stress is applied by crimping or rolling in a segment of the metal layer on the object,        crimping is done by means of a pod whereof an internal surface is covered by the metal foam,        the metal foam is embedded over the entire surface of the metal object,        it further comprises a densification step of the metal layer during which an additional metal layer made of a third material is applied to the metal layer of the assembly,        the additional metal layer is applied by chemical or electrolytic deposit,        the additional metal layer is an additional metal foam which is embedded in the surface of the metal layer of the metal assembly by repeating the steps for placement and application of stress of the process, and        the second metal is aluminium, and the process further comprises the following steps prior to the placement step of the foam on the object:        thermally processing the aluminium object by bringing it to a temperature between 80% and 100% of the fusion temperature of the material constituting it to create and stabilise a layer of alumina alpha by allotropic transformation of the layer of oxide present on the surface of said aluminium object, and        cooling the aluminium object.        
According to a second aspect, the invention relates to a metal assembly obtained by the manufacturing process according to the invention, comprising a metal layer embedded in a surface of a metal object following an application step of mechanical stress.
Some preferred though non-limiting aspects of the metal assembly according to the invention are the following:                the first and the second metal are selected respectively from the following couples of metals:        copper, aluminium,        aluminium, copper,        nickel, aluminium,        aluminium, nickel,        iron, aluminium, and        aluminium, aluminium,        it further comprises an additional metal layer made of a third metal,        the additional metal layer is embedded in the metal layer made of the first material,        the metal object is an aluminium wire or a cable and the metal layer is copper, and        the metal object is sheet metal or an aluminium foil and the metal layer is copper or nickel.        
Other characteristics, aims and advantages of the present invention will emerge more clearly from the following detailed description, and with respect to the attached drawings given by way of non-limiting examples.